| It is apparent that excitation-contraction coupling in airway smooth muscle (ASM) is unique as it relies more heavily on agonist-induced release of Ca2+ from the sarcoplasmic reticulum (SR) and C2+ -sensitization.; Although ASM appears less reliant on depolarization and Ca2+ influx via L-type Ca2+ channels, it still expresses robut inward current. Thus, we sought to examine the role of Cl- and membrane depolarization in the context of Ca2+ mobilization and SR refilling. Furthermore, we examined the role of Cl-mediated depolarization in Ca2+-sensitization, which contributes to sustained contraction of smooth muscle.; We reported that intracellular Cl- acts as a neutralizing ionic flux during the movement of Ca2+ into and out of the SR. Furthermore, blockade of a putative SR Cl- channel with NPPB reduced SR refilling.; In our examination of Ca2+ handling, inhibition of L-type Ca2+ channels did not completely abolish SR refilling. Thus we hypothesized that depolarization, in conjunction with cation influx, through store/receptor-operated channels, may trigger voltage-dependent Ca 2+ influx via the reverse-mode of the sodium-calcium exchanger (NCX). Selective inhibition of the reverse-mode of the NCX greatly reduced SR refilling. Furthemore, concurrent inhibition of the NCX and L-type Ca2+ channels nearly abolished SR refilling, suggesting both Ca2+ influx pathways were required.; We also reported a role for Cl--mediated depolarization agonist-induced Ca2+-sensitization. Chloride channel blockade significantly reduced the magnitude of sustained contractions which was associated with a decrease in the activation of RhoA and its translocation to the plasmalemmal region.; Although not contributing directly to agonist-induced contraction, our data suggest that membrane depolarization, primarly mediated by Cl -, is required to trigger Ca2+ influx for SR refilling via L-type Ca2+ channels and the reverse-mode of the NCX. Furthermore, Cl--mediated depolarization augments receptor-mediated Ca 2+ sensitization contributing to sustained contractions. In addition to depolarization, intracellular Cl- acts as a neutralizing ionic flux during Ca2+ handling. |
